Japanese Unexamined Patent Publication No. 2-66317 discloses a sliding bearing, in which an alumite layer with an uneven surface is formed on the surface of a lining consisting of aluminum alloy, a bonding layer having an uneven surface, which correlates with the uneven surface mentioned above, is formed on the lining layer, and a Pb overlay is formed on the bonding layer. It is described in this publication that, when the alumite layer is formed on the surface of lining material, the lining surface is slightly etched to form an uneven surface on the lining material simultaneously with the alumite formation in the alumite bath. It is further described that the thus formed unevenness is copied on the surface of the alumite layer to form relatively deep concavities. When the alumite bath is acidic, Sn, Pb and the like of the lining material are preferentially etched. When the alumite is alkaline, the aluminum matrix is preferentially etched.
In addition, the above mentioned bonding layer can be formed by wet plating by such methods as electro-plating or the like, and dry plating by such methods as sputtering, ion-plating or the like. Materials used for the bonding layer are Ni, Cu, Fe or the like alone or in the alloy. Plural plating layers of Ni plating, Cu plating or the like can be used after applying the Zn plating layer.
In the above described sliding bearing, the overlay layer attains, at the initial period of use enhancement of initial compatibility, mitigation of local contact, and reduction in coefficient of friction. Also, local wear of the overlay layer due to peeling or the like is prevented because its bonding strength is increased. In addition, if the bonding layer and the overlay layer are worn out locally to expose the convex surface of the alumite layer, good seizure-resistance and wear-resistance can be maintained.
Japanese Examined Patent Publication No. 57-10176 discloses a sliding bearing, whose bearing material consists of an alloy with a composition of from 3 to 10% of Sn, from 4.5 to 11.7% of Si, from 0.25 to 1.5% of Cu, from 0.1 to 1.5% of Ni (the total amount of Cu and Ni being from 0.5 to 2.0%), Al in balance, and, a 5 to 30 .mu.m thick anodic oxidation coating formed to enhance the compatibility and durability. The anodic oxidation in this publication is carried out by using a 10-14 wt. % sulfuric acid aqueous solution at bath temperature of from 20.degree. to 25.degree. C., using the bearing material as an anode and a lead plate as a cathode, and conducting direct current at a current density of 2A/dm.sup.2 for 10 to 30 minutes. The anodic oxidation coating replaces the conventional Pb-Sn based overlay which is deposited on the aluminum alloy with or without a replacement plating layer. The anodic oxidation coating allegedly improves the adherence.
Since the engines of automobiles are recently operated at high speed and under high pressure, the initial characteristics, such as roughness and waviness of a shaft have greatly influenced the performance and life of a bearing. Kelmet with an overlay exhibits good compatibility but its bearing material, i.e., kelmet, lacks in the sliding properties, such as wear resistance or the like. Therefore, when the initial properties of the bearing are poor, a problem in premature wear arises, which leads to abnormal wear and seizure.
On the other hand, the load resistance and fatigue resistance of aluminum-bearing alloys have been improved by enhancing the strength and hence hardness. Along with this, the compatibility of aluminum bearing alloys tend to become poor. Furthermore, since cast shafts with dispersed hard matters have been used frequently for engine shafts, the aluminum bearing alloys are worn out and roughened. This causes abnormal wear. A problem of seizure of bearings also arises.
In the light of the above circumstances, the prior art is considered.
Japanese Unexamined Patent Publication No. 2-66317 mentions a high-strength and high-hardness aluminum-bearing alloy but does not disclose a measure effective for preventing its seizure. That is, although the alumite layer present on the entire surface of the lining material exhibits improved adherence, it does not exhibit, on the other hand, good sliding characteristics, so that seizure is likely to occur when the overlay is almost totally worn out. Specifically, the seizure is likely to occur during initial or middle period of use when the alumite layer is present on the entire surface of the lining layer.
Japanese Examined Patent Publication No. 57-10176 discloses an aluminum bearing alloy of high-strength and high-hardness type. It has turned out that, since the alumite coating is formed by anodic oxidation by the use of sulfuric acid solution, the tin phase of the aluminum alloy dissolves by the anodic oxidation treating liquid. As a result of the disappearance of the tin phase, wear of the alumite coating advances so that a shaft slides on the mixed phase of alumite and lining. When this sliding occurs, the seizure becomes extremely liable to occur.